Generally speaking, a liquid crystal display device or an organic EL (Electro Luminescence) display device of an active matrix type includes: a substrate on which a thin film transistor (Thin Film Transistor; hereinafter also referred to as “TFT”) is formed as a switching element for each pixel (also referred to as “TFT substrate”); a counter substrate on which a counter electrode, color filters, and the like are formed; and an optical modulation layer, e.g., a liquid crystal layer, provided between the TFT substrate and the counter substrate.
In the recent years, it has been proposed to use an oxide semiconductor film such as IGZO (InGaZnOX) as a TFT semiconductor layer, instead of a silicon semiconductor film such as amorphous silicon. Such a TFT is referred to as an “oxide semiconductor TFT”. An oxide semiconductor has higher mobility than does amorphous silicon. Therefore, an oxide semiconductor TFT is able to operate more rapidly than an amorphous silicon TFT. Moreover, an oxide semiconductor film can be formed through simpler processes than those of a polycrystalline silicon film, and therefore permits easy application to devices which require a large area.
Patent Documents 1 and 2 describe examples of oxide semiconductor TFTs.
The oxide semiconductor TFT of Patent Document 1 is an oxide TFT including a semiconductor layer whose main component is zinc oxide. According to Patent Document 1, its production method includes a step of forming on a substrate a thin-film oxide semiconductor layer whose main component is zinc oxide, a step of forming a first insulating film on the thin-film oxide semiconductor layer, and a step of forming a second insulating film on the first insulating film, such that the first insulating film is oxidized before forming the second insulating layer.
The oxide semiconductor TFT of Patent Document 2 includes a thin-film oxide semiconductor layer whose main component is zinc oxide (ZnO) and which is disposed between a source electrode and a drain electrode, and a gate insulating film composed of a silicon-type insulating film which covers the upper face and side faces of the thin-film oxide semiconductor layer, such that the gate insulating film includes a first gate insulating film which covers the upper face of the thin-film oxide semiconductor layer and a second gate insulating film which covers the front face of the first gate insulating film and the side faces of the thin-film oxide semiconductor layer.